1. Field of the Invention
The present invention relates to Frequency Shift Key (FSK) Demodulators and more specifically to those FSK demodulators applicable to MOS-LSI design which are capable of near theoretical performance.
2. Description of the Prior Art
Frequency Shift Keying is a form of frequency modulation in which the modulating wave shifts the carrier frequency back and forth between first and second distinct frequencies to designate either a MARK or a SPACE. These characteristics make this form of frequency modulation particularly adaptable to digital binary systems wherein a MARK represents a digital ONE and a SPACE designates a digital ZERO. The present invention relates to FSK demodulators for detecting the first and second frequencies in the output signal.
The present FSK detection technology includes analog circuitry wherein the FSK analog signal is introduced to a discriminator wherein the frequency of the FSK signal is compared against a standard frequency. Deviations from the standard frequency are converted into amplitude variations. The standard frequency is typically the average of the first and second FSK frequencies so that the amplitude variations are positive for the first FSK frequency and negative for the second FSK frequency.
In another type of analog FSK demodulator, the FSK signal is simultaneously introduced to a pair of circuits which are respectively tuned to the first and second FSK frequencies. The signals at the output of these tuned circuits are introduced to respective envelope detectors which provide output signals having relative magnitudes dependent upon the first or second FSK frequencies in the FSK signal.
Both of these FSK detection techniques involve analog circuitry. Neither of these systems is directly adaptable to the metal-oxide-semiconductor-large-scale integration technology since elaborate and costly digital processing circuitry would be desirable in order to obtain comparable performance.
Existing digital techniques which are applicable to MOS-LSI design perform poorly relative to that which is theoretically obtainable.